Solvent-based coating compositions are known which employ high molecular weight (e.g. 5,000 to 40,000) polymer resins having crosslinking functionality, and a suitable crosslinking agent. Typically, such coating compositions are applied to a substrate, for example, by spraying, and are then cured by baking the coated substrate at an elevated temperature suitable to drive off the organic solvent and to promote the crosslinking reaction. The resulting thermoset coating, if sufficiently humidity and solvent resistant, can provide aesthetic and functional advantages including corrosion protection for the underlying substrate.
Coating compositions comprising such high molecular weight polymer resins typically comprise only 25% to 50% solids so as to be sprayable or otherwise conveniently applicable to a substrate. The viscosity of coating compositions of higher solids content is typically too high for this purpose. Conventional epoxy ester based automotive vehicle spray primers, for example, typically have a volatile organic content ("VOC") of approximately 623 g/l (5.2 lb./gal).
Elimination of the volatile organic solvent portion during curing of these conventional low-solids coating compositions is relatively large and therefore presents undesirable material handling difficulties, and added expense. Furthermore, excessive solvent losses and/or solvent recovery equipment add considerable expense to the coating operation. Recently, governmental regulations on hydrocarbon emissions, particularly applicable to automotive coating operations, mandate a significant reduction in volatile organic content for coating compositions. Thus, for example, in the United States, governmental guidelines establish certain deadlines by which time emissions of volatile organics from automotive vehicle primer coating operations must be reduced to within certain defined limits. To meet such guidelines, coating compositions of reduced VOC can be employed in conjunction with emissions treatment equipment to achieve the specified emissions limit. Such treatment presents significant additional expense, however, and thus there is a great need for coating compositions of VOC reduced near to, or preferably even lower than, the governmental limits, which yet can be applied to a substrate using known spray application techniques.
In response to these concerns, high solids coating compositions have been suggested which, typically, employ a low molecular weight multi-functional adduct or copolymer in combination with a multi-functional crosslinking agent. These high solids coating compositions can be applied by spraying, for example, with lower VOC than would be possible with conventional epoxy ester based coating compositions or other conventional coating compositions comprising high molecular weight polymer resins. After application to the substrate, high solids coating compositions are cured by baking at a cure temperature, that is, at an elevated temperature suitable to drive off the volatile organic content and to promote crosslinking and in some instances polymerization of the multi-functional low molecular weight component(s).
Typically, the physical properties of the coatings provided by such known high solids coating compositions can differ significantly from those of the cured coatings provided by the conventional, low solids coating compositions. In particular, the cured coatings obtained from known high solids coating compositions can be inferior in that they can be less flexible, less solvent resistant, less adherent to the substrate and/or for other reasons provide less corrosion inhibition for the underlying substrates.
Still further many high solids coating compositions, in particular prior art epoxy ester amino resin primers, exhibit poor package stability, especially in the presence of corrosion inhibiting pigments such as zinc chromate. Accordingly, it would be highly desirable to provide a coating composition comprising low molecular weight materials suitable for use in high solids, solvent based coating compositions and yet which, not only exhibits good package or shelf stability even in the presence of corrosion inhibiting pigments, but also, upon curing, form coatings having physical properties comparable to those obtained from conventional low solids solvent-based coating compositions. Accordingly, it is an object of the invention to provide novel coating compositions which comprise crosslinkable epoxy ester resins and blocked polyisocyanate crosslinking agent and which provide high crosslinking efficiency and hard, well cured films at minimum bake requirements such as apply to automotive primers. In this regard, it is a particular object of the invention to provide a novel epoxy ester/blocked polyisocyanate thermosetting coating composition of sufficiently low VOC to meet governmental guidelines and yet which can be applied to a substrate by spraying or other known method.
It is another object of the invention to provide a composition which will form a coating on substrate, which coating has advantageous physical properties including, for example, humidity and solvent resistance and corrosion protection for the underlying substrate. Additional aspects and advantages of the invention will be apparent from the following description thereof.